The present invention is generally related to telecommunications systems, and more specifically is related to linked compressor-expander systems (Lincompex) implemented through the use of a digital signal processor. The present invention is an improvement over U.S. Pat. 4,271,499 to J. Howard Leveque.
Lincompex systems have generally been known since the early 1960's. Basically, Lincompex has been used in HF radio communication networks to significantly improve standards of quality and stability approaching those of cable and satellite systems. The basic principles of Lincompex are as follows. Incoming speech signals are split into speech and control paths. In the control path, the envelope of the input speech level is detected and a signal is generated proportional to this detected level. A compressor circuit in the speech path uses the signal from the control path to adjust its gain according to the detected speech level at a syllabic rate so as to output speech signals of substantially constant and compressed amplitude. The control path signal is converted to a logarithmic signal to compensate for the volts to decibels conversion and the logarithmic signal is applied to a voltage controlled oscillator which produces an output frequency related to the amount of speech signal compression conducted upon the corresponding syllable. The compressed speech signal and the control frequency signal are then combined, amplified and input to a transmitter.
At the receiver, the demodulated signal containing both the speech and control frequency components is filtered and separated into speech and control signal paths. The control signal frequency is detected and passed through a logarithmic to linear network to reattain the speech envelope level signal. This signal is then applied to an expander circuit in the speech path which amplifies the compressed speech signal to the original speech input signal.
Historically, Lincompex systems have been analog in nature. The analog systems are relatively large in size and expensive and require a substantial amount of periodic adjustment which would require radio operator support. The analog circuitry is complex in nature and must be designed to operate precisely over large temperature variations, vibration, and must remain within strict parameter limits so as to be compatible with other units.
Also known in the art is a digital Lincompex system as described by the above referenced Leveque patent wherein the attenuation function of the comressor is provided by a digital circuit driven by a digital control signal. The voltage controlled oscillator is incrementally varied by a digital control signal to produce a sinusoidal output control signal. The digital control signals which control the compressor and control frequency oscillator circuits are derived from a digital signal which is representative of the detected amplitude of an input speech signal.
While the known digital Lincompex system as embodied by U.S. Pat. No. 4,271,499 represents a significant improvement over the analog design in terms of improved S/N ratio, fewer calibration adjustments, and wide temperature range of operation, the hardwired digital system still requires the use of various analog circuitry for filtering, envelope detection, control tone generation, and signal mixing, which analog circuitry still suffers from noise interference, significant power dissipation, relatively large size, and limited stability and reliability.